The basic function of any blown film extrusion die is to take one or more melt streams entering the die and distribute them to a single concentric annular melt stream at the die exit as uniformly as possible.
A number of different types of extruder die are known in the art. Concentric helical mandrel dies are cylindrical in shape and are mounted one above another secured to a common component to maintain their relative positions. Variations of this design may have, for example, a central feed where all melt streams are fed to the centre and then split to the helical outlet channels with radially arranged tubular ports. An example of this type is shown in U.S. Pat. No. 3,966,377. Another variation is an annular feed mandrel die in which the melt streams flow into the die through centrally located concentric annuli and then, through outwardly radially extending tubular ports, to helical outlet channels. This variation can be implemented in two sub-variations; one with central part of the feed block is open for IBC tube installation, the other having the inner layer stream as the central pipe of the feed block. Examples are to be found in U.S. Pat. No. 4,182,603. A further variation is a side-feed mandrel die in which each layer has a single melt inlet on the outside of the die and the melt is then distributed in a pair of vertical paths to the entries of the helical outlet channels, each of the two distribution paths being on the same cylindrical surface as the corresponding helices. An example is to be found in U.S. Pat. No. 7,811,073.
Conical stacked mandrels stack one over another, with a variation of this design consisting of conical shaped mandrels stacked over one another outside and/or inside a vertical common path. Several options also exist as to the feeding of the helical entries of these dies, e.g. central feeding of all layers or side feeding of the layers with horizontal split feed at different heights. Examples are to be found in U.S. Pat. No. 6,702,563
Modular plate mandrels are split two-part modules which, like some conical designs, stack one on another. There are basically two options; out-in versions in which the melt streams flow from outside to inside, and in-out versions in which the melt streams flow from inside to outside of each module. A combination of the two types is also possible. The melt distribution is typically horizontally split but it can also combine some vertical paths to reduce diameter.
It is also possible to combine the types, for example the basic die being of concentric mandrel type with some layers being of modular plate design, usually for the outer layers of the multi-layer blown film.
WO01/78966 discloses a co-extrusion die with one of the extruded components being fed through a single side inlet into a bifurcated feed channel which supplies the die outlet and similar constructions are shown in WO90/11880 and JP2011005824.
US2004/022886 discloses a single layer extrusion die with side inlets feeding multiple bifurcated feed channels.
FIG. 1 attached shows a five-layer concentric mandrel die 1 with central feeding in accordance with prior art. The die shown in FIG. 1 has an 1800 mm diameter, i.e. for the annular path 110 to which all layers flow after they merge. This diameter also corresponds to the diameter of the mandrels of the middle layer of the die. Since the die is of centre-fed type, all the extruder inlets 401-405 (only two of which—401 and 405 are shown for simplicity) through which the molten thermoplastic is fed to the die are placed close to the bottom of the die and are spread around the perimeter. As can be seen, such dies have complex internal constructions, requiring the accurate registration of components in the different layers.
To avoid complexity in the figures only the main elements of parts of the flow paths are shown, but in detail each flow path includes:                a horizontal inlet part 401-405, which extends to the centre of the die for the outer layer and towards the centre of the main die body 10, but to an off-center point for the remaining layers.        a path 501-505 directed upwards and making any necessary bends in order to avoid collision with other layers and reach the centre (only path 501 is shown—for simplicity).        multiple inclined radial ports 601-605 (which may also be horizontal and only port 601 is shown). Each of the inclined ports also has an additional vertical path as soon as it arrives at the layer mandrel (the one indicated in the drawing is small but still exists).        
The length of each of the flow paths for the layers 201-205 from the die inlet to helical outlet channel start point is, for the layers in turn from inner layer to outer layer, 2772 mm, 2776 mm, 2803 mm, 2834 mm and 2893 mm.
Blown film dies exist from sizes of 50 mm to 2500 mm diameter. Most of the dies that are used for packaging film applications are of a maximum diameter of 900-1000 mm and up to eleven layers. These dies are of either concentric mandrel, modular or conical, or mixed. Conical stacked mandrel and modular plate designs can be implemented up to a diameter size of 900 mm, flared from 600-700 mm. There exist dies of modular plate design which are flared to 1300 mm from 600-700 mm and which have very long flaring melt flow paths.
Larger dies (up to 2500 mm) are typically of three to five layers and are usually central or annular mandrel dies, typically used for agricultural applications (e.g. greenhouse films) where large film dimensions are necessary (e.g. 8 to 22 m bubble circumference, 100-200 μm thickness), or for geomembrane applications (6 to 8 m bubble circumference, 500-2500 μm thickness)
In existing concentric dies of side feed design, the material follows a binary split distribution feed channel arrangement from a single side inlet of the layer to the starting point of each extrusion helical outlet. As the die gets bigger in diameter, the length of this flow channel gets longer and longer. As a consequence, higher melt pressures are developed in use and the material residence time gets longer, resulting in increased melt temperatures and material degradation. As a result side feed concentric mandrel dies have been limited to about 1200 mm die diameter.
In a typical large blown film die having 3 to 5 layers and 1800 mm die diameter all layers are centrally fed. For the middle layer of the die, this results an overall length between the die inlet to the helical outlet of more than 2700 mm (see above reference to FIG. 1).
As a consequence the average residence time as well as the tail of the residence time distribution become very long. In addition the size of the die does not allow reducing the thickness of a specific layer while maintaining good thickness uniformity of this layer due to the very long paths that the melt has to flow though within the die and the required low material quantity for such layer. Further, the back pressure developed between the layer inlet and the start of the helical outlet becomes very high, reducing the remaining available pressure which can be used for the helical outlet section of the layer to improve thickness uniformity due to the fact that total available pressure is limited.
Residence time and melt distribution around the die circumference is also a very critical issue for large dies, especially for sensitive materials because of carbon build up, high purging time, waste, deposits, etc. (slow moving particles are prone to degradation and long purging time).
An example is a 2 m diameter, five layer concentric mandrel die where the middle layer is designed to extrude an ethylene vinyl alcohol copolymer (EVOH) film at a very low output and percentage (e.g. less than 4%) and at a very good thickness tolerance distribution around the die circumference.
Such materials need to be processed with a very short residence time and also need to be used in very low percentages due to their significantly higher cost in relation to standard materials. As an example, EVOH has a cost which is in the range of 5-6 times higher compared to Polyolefins, therefore in case of a film combining both materials, EVOH has to be used in small percentages in order for the film to be of reasonable cost while maintaining the advantage that using EVOH has as to the barrier properties it provides.
Thus, it is often desirable to reduce residence time distribution, to minimize wetted surface area (the area where the polymer comes to contact with the metal), to minimize the melt volume inside the die, to optimise back pressure, to avoiding overheating the die, to enable rapid purging for efficient product change-over and reduction of resin waste, to eliminate flow lines in the final products, to eliminate melt fracture, interfacial instability, gels, black spots, carbon built up, etc., to improve operational flexibility in resin selection and processing parameters, to increase output levels and/or efficiency, to improve thickness tolerance of each layer and total film thickness, to improve film optics, and to achieve thermal isolation between layers especially the ones with significantly different processing temperatures.
The present invention targets, in particular, large co-extrusion blown film dies (with mandrel diameter above 1200 mm) for producing film bubbles of large circumference (8 to 22 m).